1. Field of the Invention
The present invention relates to a method of correcting the light quantity of an optical writing apparatus, said apparatus being used for digital printers and the like.
2. Description of the Prior Art
As a solid-state scanning optical writing apparatus, an array type using optical output media such as LEDs, PLZT, fluorescers and liquid crystal, etc. aligned in one direction has conventionally been known.
In optical output media used in the conventional solid-state scanning optical writing apparatus, optical outputs such as light emission and light transmission vary among pixels. This variation is generally a combination of a random variation and a periodical variation. FIG. 14 shows an example of the variation, among pixels, in the quantity of transmitted light of a print head using PLZT as a light quantity distribution of the optical outputs. As is apparent from FIG. 14, the light quantity variation conditions in the gradation ranges are substantially analogous.
Conventionally, to correct such a variation in the light quantities of the optical outputs, normally, with the darkest pixel or region in the solid-state scanning optical writing apparatus as the reference, the output levels of other bright pixels are reduced so that the light quantity levels of all the pixels are made uniform. For example, the highest output level of the light quantity distribution in the gradation range shown by a in FIG. 14 is 25000 a.u., whereas the lowest output level thereof is 15000 a.u. The output level variation is made uniform by the above-described correction so that the output levels of all the pixels are substantially 15000 a.u. which is the lowest output level as shown by a1 of FIG. 15. The highest output level of the light quantity distribution in the gradation range shown by b in FIG. 14 is 20000 a.u., whereas the lowest output level thereof is 12500 a.u. The output level variation is made uniform by the above-described correction so that the output levels of all the pixels are substantially 12500 a.u. which is the lowest output level as shown by b1 of FIG. 15. Likewise, the light quantity distribution in the gradation range shown by c in FIG. 14 is made uniform to the output level condition shown by c1 in FIG. 15 by the above-described correction. The light quantity distribution in the gradation range shown by d in FIG. 14 is made uniform to the output level condition shown by d1 in FIG. 15 by the above-described correction. The light quantity distribution in the gradation range shown by e in FIG. 14 is made uniform to the output level condition shown by e1 in FIG. 15.
Since the optical output variation among the optical output medium differs according to the color of the light, when optical writing of color images is performed, a similar correction is performed for each of the three colors of R, G and B.
When optical writing is performed, for example, onto silver halide photographic paper by the solid-state scanning optical writing apparatus while performing such an optical output correction, an output density is obtained where there is no influence of the variation in the light quantities of the optical outputs of the optical output media as shown in FIG. 16. The density distribution shown by a2 in FIG. 16 corresponds to the gradation range shown by a1 in FIG. 15. The density distribution shown by b2 in FIG. 16 corresponds to the gradation range shown by b1 in FIG. 15. The density distribution shown by c2 in FIG. 16 corresponds to the gradation range shown by c1 in FIG. 15. The density distribution shown by d2 in FIG. 16 corresponds to the gradation range shown by d1 in FIG. 15. The density distribution shown by e2 in FIG. 16 corresponds to the gradation range shown by e1 in FIG. 15.
However, in the above-described conventional correction, the larger the output level variation in a head is, the more of the steps of gradation of all are sacrificed. For example, the average output level at the light quantity distribution in the gradation range shown by a in FIG. 14 is approximately 23000 a.u. and this is reduced to 15000 a.u. by the above-described correction, so that only approximately 65.52% of the actual gradation range can be used. Consequently, the number of steps of gradation is reduced, so that the substantial contrast which is the ratio between the highest output light quantity and the light quantity at the time when the apparatus is off is reduced. Moreover, a so-called pseudo gradation occurs in which gradation joints appear in images. Thus, the above-described correction has problems in the reproducibility of multi-value images such as color images.